Johne's disease is a potentially deadly condition in cattle usually caused by Mycobacterium avium subsp. paratuberculosis, which is abbreviated as MAP. Due to the deficiency of a practical diagnostic test for MAP, the estimation of actual prevalence and economic losses caused by Johne's remained in darkness, despite the significant impact of this disease in all domestic ruminant industries worldwide. The infection happens in the first few months of an animal's life but the symptoms usually develop after a couple of years.
Two major problems are associated with control of Johne's disease. First, for every clinically affected animal in a herd, there are usually dozens of animals which are infected but not showing any symptoms. Consequently, these subclinical carriers are the major source of further transmission of the disease. The second problem is that there is no rapid, reliable, inexpensive test for the identification of these carriers. Currently, the method of choice is the bacterial culture of feces on specialized media. Unfortunately, growing MAP on artificial media is difficult. Further reducing the ability to easily identify carriers of the disease is that the fecal microbial population in an animal varies with time, and thus, repeated culturing attempts are necessary before an infected individual can be identified. Techniques such as Enzyme-Linked Immunosorbent Assay (ELISA), biochemical tests and/or polymerase chain reaction (PCR) have been considered but there are limitations with each technique. ELISA is impeded by antibody degradation. PCR-based strategies are more reliable, but their reliability is still greatly limited by inhibition issues caused by interfering agents, such as fatty acids and calcium ions. Therefore, a need exists for improved techniques and methods for the diagnosis of Johne's disease.